The Hewlett-Packard 47201A ear oximeter and the Radiometer TCM, transcutaneous oxygen monitor were evaluated for use as noninvasive monitors of arterial oxygenation during inhalational anesthesia in man. Thirty-four healthy adult volunteers were anesthetized to steady states with halothane, enflurane, or isoflurane, and were studied either before or during surgery. Oxygen levels were varied over ranges that included hypoxemia, by manipulating Fio2. Oximeter estimates of Sao2 values and transcutaneous estimates of Pao2 values were compared with conventional measurements of each. Oximeter readings responded rapidly to changes of inspired oxygen concentration and were acceptably accurate estimates of Sao2, except at lower Sao2 levels (2 and were frequently inaccurate reflections of Pao2 values. We consider this oximeter suitable as a monitor of arterial oxygenation during anesthesia, but find the transcutaneous electrode unsatisfactory.